In this post, I will introduce lvalues and rvalues in C\/C++. But I leave rvalue references and move semantics to the next post.<\/p>\n
An assignment expects an lvalue as its left operand. See the codes below.<\/p>\n
\/\/ lvalue in left operand\nint var;\nvar = 4;\n\n\/\/ rvalue in left operand\n4 = var; \/\/ ERROR!\n(var + 1) = 4; \/\/ ERROR!\n<\/code><\/pre>\n2. Function can return lvalue or rvalue<\/h4>\n
We can assign value returned by a function. See the codes below.<\/p>\n
\/\/ error: lvalue required as left operand of assignment\nint foo() {return 2;}\n\nint main()\n{\n foo() = 2;\n\n return 0;\n}\n\n\/\/ good \nint globalvar = 20;\n\nint& foo()\n{\n return globalvar;\n}\n\nint main()\n{\n foo() = 10;\n return 0;\n}\n<\/code><\/pre>\nActually []<\/code> in map<\/code> works by using this.<\/p>\nstd::map<int, float> mymap;\n\/\/ [] returns a reference that can be assigned to\nmymap[10] = 5.6;\n<\/code><\/pre>\n3. Unmodifiable lvalues<\/h4>\nconst int a = 10; \/\/ 'a' is an lvalue\na = 10; \/\/ but it can't be assigned!\n<\/code><\/pre>\n4. Conversions between lvalues and rvalues<\/h4>\nImplicit lvalues-to-rvalues<\/h5>\n
Language constructs operating on object values require rvalues as arguments. All lvalues that aren’t arrays or functions can be converted to rvalues.<\/p>\n
int a = 1; \/\/ a is an lvalue\nint b = 2; \/\/ b is an lvalue\nint c = a + b; \/\/ + needs rvalues, so a and b are converted to rvalues\n \/\/ and an rvalue is returned\n<\/code><\/pre>\nExplicit lvalues-to-rvalues<\/h5>\n
The unary address-of operator &<\/code> takes an lvalue argument and produces an rvalue.<\/p>\nint var = 10;\nint* bad_addr = &(var + 1); \/\/ ERROR: lvalue required as unary '&' operand\nint* addr = &var; \/\/ OK: var is an lvalue\n&var = 40; \/\/ ERROR: lvalue required as left operand\n \/\/ of assignment. \n<\/code><\/pre>\nExplicit rvalues-to-lvalues<\/h5>\n
Implicit rvalues-to-lvalues is not possible. But we can use things like unary operator *<\/code> (dereference), which takes an rvalue argument but produces an lvalue as a result.<\/p>\nint arr[] = {1, 2};\nint* p = &arr[0];\n\/\/ good: p + 1 is an rvalue, but *(p + 1) is an lvalue\n*(p + 1) = 10; \n<\/code><\/pre>\n5. Lvalue references<\/h4>\n
&<\/code> can produce “lvalue references”. It takes a lvalue and return a lvalue reference. Non-const lvalue references cannot be assigned rvalues, since that would require an invalid rvalue-to-lvalue conversion.<\/p>\nstd::string& sref1 = \"Hello world!\"; \/\/ ERROR: invalid initialization of non-const reference of type 'std::string&' from an rvalue of type 'std::string'\nconst std::string& sref2 = \"Hello world!\"; \/\/ good \nstd::string s(\"Hello world!\");\nstd::string& sref3 = s; \/\/ good \n<\/code><\/pre>\nReference<\/h2>\n